My road tires are 700 X 25 and I pump them up to 100 (max is 120). The wider the tire the lower the pressure can be. 28's - 32's can run around 70 - 90 and mountain bike tire generally run around 40. It amazes me some of the really thin tires - 21's and 23's can be blown up to 150 or higher - get a puncture and KAPOW-Y!!!!!!